The invention relates to the visual display of information representing a variable quantity over an area having parts with distinct properties and especially, but not exclusively, to displaying x-ray images, including tomographic images, of a body part including both hard and soft tissue.
The display of information in the form of images is well known. Such images may display almost any property that has a magnitude varying over an area in the form of an image in which color or grayscale shade is used to represent the local value of the property. For example, in x-ray images, grayscale shade commonly represents tissue density, as measured by the ability of the tissue to absorb x-rays.
A set of three-dimensional data relating to a property of an object that varies over space within the object may be obtained in various ways. For example, an x-ray image of a target may be obtained by placing the target between a source of x-rays and a detector of the x-rays. In a computed tomography (CT) system, a series of x-ray images of a target are taken with the direction from the source to the detector differently oriented relative to the target. From these images, a three-dimensional representation of the density of x-ray absorbing material in the target may be reconstructed. Other methods of generating a three-dimensional dataset are known, including magnetic resonance imaging, or may be developed hereafter. From the three-dimensional data, a tomogram, which is a section in a desired plane, may be generated.
However, in a typical x-ray image both soft and hard tissues may be visible. When only one form of tissue is of interest, detail in the other form can be an unnecessary and undesirable distraction. For example, in a dental x-ray, on many occasions only the teeth and bones of the patient are of interest. In displaying an electronic x-ray image or tomogram, the grayscale can be adjusted so that the soft tissue becomes a very dark gray background. That background is typically not uniform, and the patterning on the background may be distracting.
In addition, the soft tissue image often contains significant noise, and very little actual small scale structure. Consequently, if it is desired to examine the soft tissue, the image can often be improved by removing fine detail from the soft tissue parts of the image. In the hard tissue, such as bone and teeth, fine structure is often both real and important, so it is preferred to preserve the fine detail of the image.
There is therefore a hitherto unfulfilled need for a system by which within every image, noise is removed from the soft tissue parts of an image, while retaining the crispness of imaging of small high density objects within the same image.